The present invention relates to lift trucks, and more particularly, to lift trucks having two-stage primary masts.
In most warehouses, and especially in warehouses of the type which store palletized goods, profitability of the warehousing operation can be increased by maximizing the useage of available floor space. Accordingly, such warehouses are designed to make the aisles between racks as narrow as possible, and to make the racks upon which the pallets are stored as high as possible, thereby increasing the number of pallets stored per unit of floor space.
In response to this trend in warehouse design, it has been necessary to design lift trucks which are sufficiently narrow in width to negotiate relatively narrow aisles, yet have the capability of placing a load on a rack which may be as high as 40 feet (12.2 m) above the floor of the warehouse. Lift trucks of this type may comprise a frame having a rearwardly mounted power unit and forward load wheels, a primary mast having a mainframe mast extending upwardly from the frame, and a second stage mast mounted to and movable relative to the mainframe mast. A main carriage is mounted to and movable relative to the second stage mast, and preferably includes a platform with an operator's station and a swivel-mounted lifting fork which in some models is carried by an auxiliary mast attached to the platform. Thus, the operator as well as the lifting forks can be elevated to the appropriate level so the operator can direct the forks to deposit or retrieve items from a shelf or bin which is elevated above the floor.
One problem inherent in such devices is that, when fully extended, the lifting masts become less stable in response to bending moments or torsional loads. Bending moments of a relatively large magnitude occur when a heavy pallet is placed on the forks, and can be in a front-to-back or longitudinal direction when the pallet is in front of the truck, and in a side-to-side or transverse direction when the pallet is to the side of the truck, such as during loading or unloading operations. Torsional stresses may result from the movement of the lift truck or relatively rapid movement of the lifting forks when supporting a pallet of relatively heavy articles. Such loads may cause front-to-back, side-to-side, and/or twisting motion of the primary mast. Accordingly, it is necessary to design the primary masts of such lift trucks to be sufficiently rigid to withstand bending moments and torsional loads, yet possess economy of material and design to minimize fabricating costs and weight.
One example of such a lift truck is disclosed in the van der Laan U.S. Pat. No. 3,937,346. That patent discloses a lift truck having a frame to which is mounted a vertically oriented outer mast that supports an inner mast for vertical movement by a pair of hydraulic cylinders. Each of the masts comprises a pair of parallel, vertically extending channel beams, located at a forward portion of the mast, and a box beam located at a rearward portion and joined to the channel beams by a plurality of transverse beams spaced along the length of the mast. Each channel beam defines a track, and includes rollers which are positioned to engage the track of an adjacent beam of the other mast to provide rolling engagement between the inner and outer masts. The box beams each include guide plates which extend along its length to provide a guide surface against which the other box beam slides to resist transverse bending moments which would tend to bind the rollers in the tracks.
A platform, having an auxiliary mast with turret-mounted forks, is supported by rollers between the tracks of the inner movable frame, and is displaced relative to the inner frame by hydraulic cylinders. Thus, the operator, seated in the platform, can be elevated to the top of the inner mast, and the inner mast elevated relative to the outer mast to place the operator at elevated positions where he can manipulate the forks to deposit or retrieve loads on various shelves of a storage rack.
A disadvantage of this device is that the open framework of the masts is inherently susceptible to twisting in response to torsional stresses. As a result, it is necessary to include the massive, expensive and heavy box beams and transverse beams having a rectangular channel construction to strengthen the masts. While the guide plates help resist transverse bending movements, they add an undesirable element of sliding friction to the movement of the masts, thereby increasing the power required to lift the inner mast.